1. Field of the Invention
The present invention relates to a Y/C separating circuit suitably for use with NTSC system and PAL system and for precisely separating a luminance signal and a chrominance signal from an input composite picture signal with small data correlation between horizontal lines of the input composite picture signal.
2. Description of the Related Art
FIG. 9 is a block diagram showing a construction of a conventional separating circuit that is used with the NTSC system and separates a luminance signal and a chrominance signal from an input composite picture signal. Referring to FIG. 9, the input composite picture signal is supplied to a band-pass filter BPF 52 and a one-horizontal-period (1H) delaying circuit 50. An output signal of the delaying circuit 50 is supplied to a band-pass filter BPF 53 and a one-horizontal-period (1H) delaying circuit 51. An output signal of the delaying circuit 51 is supplied to a band-pass filter BPF 54.
The band-pass filters BPF 52, 53, and 54 extract chrominance signal components of these horizontal lines. These output signals that are output from the band-pass filters BPF 52, 53, and 54 are referred to as H0, H1, and H2, respectively. The signal H1 (=b) is referred to as present data. To match a chrominance subcarrier phase of the signal H1 with a chrominance subcarrier phase of each of the signals H0 and H2, the phases of H0 and H2 are inverted by inverters 58 and 59 and output as signals a and c, respectively. Adders 56 and 57 extract signals d and e, where d=(a+b) /2 and e =(b+c)/2. The signals a, b, c, d, and e are supplied to a five-tap median filter 60 that selects a signal with third largest signal level and outputs it as a chrominance signal (C). A subtractor 15 subtracts the chrominance signal (C), which is output from the median filter 60, from an output signal of the 1H delaying circuit 50 and outputs a luminance signal (Y). However, when the correlation of data between horizontal lines of an input composite picture signal is small, the median filter 60 may not extract a required chrominance signal. In this case, a dot interference takes place in the luminance signal (Y), which has been generated by subtracting the output of the median filter 60 from the output of the delaying circuit 50.
The construction of a Y/C separating circuit for use with the PAL system is the same as that for use with the NTSC system except that delaying circuits 50 and 51 delay respective input signals by 2H instead of 1H. As with the Y/C separating circuit for use with the NTSC system, when the correlation of data between horizontal lines of an input composite picture signal is small, a dot interference takes place in a luminance signal (Y), which has been generated by subtracting the output of the median filter 60 from the output of the delaying circuit 50.
Next, with reference to FIGS. 10A and 10B, the dot interference will be described.
FIG. 10A shows a pattern where a red chrominance horizontal line is present at the center of a white screen. This pattern is photographed by a camera or the like and extracted as a composite picture signal. Thereafter, a luminance signal and a chrominance signal are separated from the composite picture signal by the Y/C separating circuit as shown in FIG. 9. When the resultant signals are displayed on a screen of a television receiver, a pattern as shown in FIG. 10B appears. This is because chrominance signal components remain in the luminance signal separated by the Y/C separating circuit shown in FIG. 9 and the chrominance signal components are displayed in a dot shape on the screen of the television receiver. In other words, such a conventional Y/C separating circuit cannot satisfactorily separate a luminance signal and a chrominance signal from an input composite picture signal.
Next, with reference to FIGS. 11 and 12, the dot-interference that takes place in the conventional Y/C separating circuit will be described in more detail.
FIGS. 11 and 12 schematically show a chrominance signal equivalent wave form chart and a luminance signal equivalent wave form chart for the NTSC system (14 lines of picture signal) and the PAL system (27 lines of picture signal). A horizontal scale of 10 of each chart represents one horizontal line period. In FIG. 11, sine waves with (n+1/2) .lambda. (where .lambda.=wavelength; n=any integer) are present in one horizontal line period. In this chart, a chrominance signal is simply illustrated. In FIG. 12, sine waves with n.lambda. are present in a horizontal scale of 10 (one horizontal line period). In this chart, as with FIG. 11, a luminance signal is simply illustrated. In these charts, H0, H1, and H2 represent output signals of band-pass filters 52, 53, and 54, respectively. When it is assumed that the signal H1 is a present signal, the signals H2 and H0 are a past signal and a future signal, respectively. In FIG. 11, in line 13 where only the signal H1 is a chrominance signal and in line 14 where only the signal H1 is not a chrominance signal, the output signal of the median filter 60 differs from the present signal H1. Thus, a dot interference (one-line-dot interference) takes place in the luminance signal (Y), which has been generated by subtracting the output signal of the median filter 60 from the input composite picture signal.
As described above, regardless of the NTSC system and the PAL system, when the correlation between the present horizontal data of the input composite picture signal and each of the past horizontal data and the future horizontal data (spaced apart therefrom by one horizontal period in the case of the NTSC system and by two horizontal periods in the case of the
system) is small, a line-dot interference takes place in the luminance signal separated from the input composite picture signal, thereby deteriorating the picture quality of the television receiver.